Limiting amplifier



March 17, 1942.

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ATTORNEY.

March 17, 1942. M. G. CROSBY LIMITING AMPLIFIER Filed May 23, 1939 2 Sheets-Sheet 2 VOLTAGE Eg I NV EN TOR. MUR/Ayacposy A TTORNE Y.

Patented Mar. 17, 1942 2,276,565 LIMITING AMPLIFIER Murray G. Crosby, Riverhead, N. Radio Corporation ol' America,

of Delaware Y., assignor to a corporation Application May 23, 1939, Serial No. 275,151

13 Claims.

The present invention relates to ampliers and, more particularly, to limiting amplifiers in which both positive and negative half cycles of the input wave are limited to a predetermined amplitude.

As far as I am aware, limiters disclosed in the prior art have been either of the negative grid cascaded direct current amplifier type or of the type which utilizes an overloaded tube with lowered element voltages. Limiters of the rst mentioned type which utilize cascaded direct current amplifiers are ordinarily directly coupled by means of bucking batteries. Thus the plate of the first stage of amplification is connected di- .ectly to the grid of the second stage through a bucking battery which keeps the plate voltage from being effective on the grid of the second stage. These bucking batteries are bulky and inconvenient from the standpoint of replacement. Furthermore, the majority of the batteries used in this type of limiter are at a high potential relative to ground. The second mentioned type of limiter which utilizes an overloaded tube depends upon negative grid cut-off to limit one-half cycle of the Wave and upon a lowered element voltage for the limiting of the other half cycle. This arrangement, as far as I am aware, usually results in a dissymmetry which produces a limiter which is not at. A limiter which is not fiat is one whose output does not remain constant after the limiting point is reached but rises or lowers. Another disadvantage which usually accompanies limiters heretofore known in the art is the fact that the input grid draws current and places a load on the input circuit above a certain input level.

An object of the present invention is to provide a limiter which is free from the heretofore mentioned disadvantages.

Another object of the present invention is the provision of a negative grid-cut-of limiter which will accurately limit both positive and negative half cycles of the input wave.

Still another object is the provision of a limiter which has stable characteristics.

A further object of the present invention is the provision of a limiter which places no load on the input circuit.

Still a further object of the present invention is the provision of a cathode drive coupling limiting amplifier which does not require batteries at a high potential above ground and which has the input characteristics of a class A amplifier regardless of the signal level applied to the input.

Further objects andV advantages will appear from the following detailed description taken in connection with the accompanying drawings and appended claims.

In accordance with an embodiment of my invention, I provide a pair of negative grid limiting amplifiers which are coupled together by a cathode drive coupling. In this way the first tube is effective for a negative grid limiting of the negative half cycles of the input signal, while the second tube, due to phase reversal of its grid-cathode voltage with respect to the gridcathode voltage of the first tube, is effective for a negative grid limiting of the positive half cycles of the input wave.

Reference will now be had, for a more complete understanding of the invention, to the following detailed description, which is accompanied by drawings, in which Figure 1 shows one specific embodiment of the invention; Figure 2 shows a modification of Figure 1 utilizing a tuned output circuit; Figure 3 is a series of curves explanatory of the operation of the circuits shown in Figures 1 and 2; Figure 4 shows afurther modification of the invention; Figure 5 is a voltage output curve showing the characteristic form of output wave from a limiter embodied in my invention; Figure 6 shows an embodiment of the invention with an alternative form of output circuit and a cathode coupling circuit, and Figure 7 is a curve showing the resultant form of output from the circuit of Figure 6.

In Figure 1 I have shown a pair of thermionic discharge tubes or electron discharge structures T1 and T2. These tubes are entirely separate as far as their electron streams are concerned but have been shown as enclosed within the same.

glass bulb. While a double triode type of amplifier tube has been shown, it is within the scope of my invention to use entirely separate triode tubes if desired. It should be further understood that I intend by the use of the term tube" lto mean a structure including a cathode electrode and an anode electrode within an evacuated space and means for causing, influencing or otherwise controlling an electron stream between said electrodes regardless of whether the evacuated space is common to more than one electrode assembly or how it is maintained in an evacuated condition. Tube T1 comprises a triode type of amplifier tube having a cathode H,

a grid I2 and a4 plate I3, while tube T2 has similar elements 2|, 22 and 23. The input to my llimiting amplifier is conventional in construction embodying a grid resistor 2 and a coupling condenser I, the grid resistor 2 being connection of the two plate circuitsto the sourceof plate potential. Plate 23 of tube Tz is coupled to any desired utilization circuit through condenser 6 by means of terminals 1, 8. While I have in this description referred to certain connections as being made to ground, it is to be clearly understood that an actual ground connection may not be necessary, the ground connection being merely a reference point of zero signal potential.

In the circuit shown in Figure 1 when the grid I2 of tube T1 is made positive increased cathode current is drawn through resistance 3 so that the cathodes II and 2l of both tubes are made more positive with respect to ground.

4Making the cathode 2l of tube T2 more positive withrespect to ground is equivalent to making the grid 22 more negative. Thus a positive change on the grid I2 of tube T1 effects a resultant negative change on the grid 22 of tube T2. This phase reversal causes tube T2 to effect the negative grid limiting for the positive haii' cycles of the input wave while tube Ti eifects it for the negative half cycles. Thus, when grid I2 of tube Ti is swung negative, negative grid cut-of! limits the change in cathode current caused by the input wave. When the grid of tube T1 is swung positive, El'd 22 of tube T2 is eectively swung negative until negative cut-oil. is reached for tube T2.

In Figure 2 I have shown a circuit which operates in substantially the-same manner as that shown in Figure 1, with the exception that a tunedl output circuit 25 has been provided in the plate circuit of tube T2 instead of the resistance 5. The tuned circuit 25 which is composed of inductance L and a capacity C, either or both of which may be variable, may be coupled to output terminals 9, I by means of acoil L' in inductive relationship with coil L. Coil L may, likewise, be tuned to the desired resonant frequency by means of condenser C'. If a tuned output circuit is not desired an impedance coupled output may be obtained at terminals 1, 3, which are coupled to the plate circuit by means of condenser 6. as heretofore described with reference to Figure 1.

In Figure 3 I have shown a curve L, L with the input grid voltage Ee as abscissae and the ,output plate current Ir as ordinates. This curve shows the static characteristic of my limiting amplier. Below curve L, L I have shown sinusoidal input voltages Ep., En and Epe, and output currents Ip., Ipb and Ipe referred to the static characteristic of curve L, L above. With no permanent bias on either grid I2 or 22 of my limiting amplifier, the amplier operates at point a of the curve. As long as the input voltage En does not exceed the straight portion of the static characteristic the plate current Ip. follows substantially the same form as the input voltage. However, after the input grid volt- `age swings beyond either knee of the characteristic curve no further change takes place in the output plate current and thus a substantially ilat topped current wave is obtained. Ii a negative bias is placed on grid I2 of tube T1 the amplifier operates about point b of the static characteristic curve and the resultant outrent curve Ipp. The negative bias on grid I2 of A tube Ti may be applied by means of a bias battery or merely by making the grid 22 of tube T2 more positive. In a similar manner, if a negative bias is applied to grid 22 of tube T2 or a positive bias on grid I2 of tube T1, my limiting amplier will operate about the point C on the static characteristic curve. The resultant plate current will then follow the form shown in curve I. The plate current curve Ipe, it will be seen, is a mirror image of the curve just previously described. By means of the diierential bias Just described it will be seen that square wave forms are obtainable with variable length mark and space, as desired.

The circuit shown in Figure 4 is particularly adapted to the production of waves of variable mark and space. such as those shown by curves Ipb and Ipe in Figure 3. This circuit differs from those previously described in the inclusion of potentiometers I4 and I5 between the grid returns of the two grid circuits and ground. With this circuit in order to obtain a wave form such as shown by curve Ipb in Figure 3, the variable arm of potentiometer I4 is adjusted to the ground or low potential end of the potentiometer so that no bias voltage is applied to grid 22 of tube Ta. The arm of potentiometer I5 is adjusted to provide a ni'te positive bias voltage to grid I2 of tube Ti. This positive voltage on grid I2 causes its cathode current to increase. The increase of current through resistance 3, thereby makes cathode voltage of both tubes more positive. The resultant eiect is to make the grid 22 of tube T2 more negative so that it is closer to its cut-off voltages. Similarly, by reversing the relative positions of the arms of potentiometers I4 and I5 so that a positive voltage is applied to grid 22, the grid I2 of tube T1 will become more negative and a wave form which is the mirror image of that previously obtained will appear in the output.

I have found that by varying the value of the cathode resistor 3 the degree of ampliiication may be varied, as well as the input at which grid current is drawn. For instance, higher values of cathode resistor 3 produce lower degrees of ampliilcation and increase the value of input voltage at which grid current is drawn. Since grid current ilow produces dissymmetry in the limiting action of-the amplifier higher values of cathode resistor are more desirable for high degrees of limiting. The over-all gain of the am,- plifier is reduced by higher values of cathode resistor due to the greater percentage of inverse feedback present on tube T2. The percentage of inverse feedback on tube Ti remains constant at percent, regardless of the value of the cathode resistor.

In Figure 5 I have shown the output voltage wave form resulting from the current Ip. owing through resistor B oi. Figures 1 and 4. It will be seen that the wave form of the voltage wave is the same as that of the current wave but the relative amplitude is much higher. Similar output voltage waves are obtained for the currents Ipb and Ipc shown in Figure 3.

If a diiierent type of output network such as shown in Figure 6 is utilized a triangular-output wave form may be obtained. The output circuit shown in Figure 6 includes a resistor 25 and a condenser 26 connected in series between condenser 6 and ground, and the square topped wave ordinarily obtained by my limiter applied thereacross. 'I'he nal output is then taken from terminals 1 and 8 across condenser 26. The resultant triangular wave is shown in Figure 7. A further modification of the circuit is shown in the cathode coupling arrangement in Figure 6. Instead of a single coupling resistor 3, as shown in the previous gures, each cathode is separately connected to ground by resistances 3 and 3'. The cathodes are coupled together by an impedance Z which may be a condenser, a resistor or an inductance. By varying the value of the coupling impedance and its character the wave form of the output wave may be varied somewhat or, in case unsymmetrical limiting is obtained by proper adjustment for the desired amplitudes of input and output, the output wave form may be made perfectly symmetrical.

While I have particularly described and shown several embodiments of my invention, it is to be clearly understood that my invention is not to be limited to these embodiments but that modicatons Within the scope of the invention may be made.

What I claim as my invention and desire to have secured by Letters Patent is:

1. A limiting amplier comprising a pair of thermionic discharge tubes each having an anode, a grid and a cathode, said cathodes being so coupled together that a variation in the discharge current of one of said tubes varies the discharge current of the other of 'said tubes, means for applying a signal to the grid of one of said tubes suihcient to swing said grid beyond cut-01T and an output circuit coupled to the anode of the other of said tubes, the remaining grid and plate being maintained at a zero signal potential.

2. A limiting ampliiier comprising a pair f thermionic discharge systems each having an anode, a grid and a cathode, means for so coupling said cathodes together that a variation in the discharge current of one of said systems thereby influences the discharge current of the other of said systems, means for applying a signal to the grid of one of said systems suiflcient to swing said grid beyond cut-off and an output circuit coupled to the anode of the other of said systems and means for independently and adjustably biasing the grids of each of said systems, the remaining anode and grid being maintained at zero signal potential.

3. A limiting amplifier comprising a pair of thermionic discharge systems each A having an anode, a grid and a cathode, an input circuit coupled from thegrid of one of said systems to a point of zero reference potential, an output impedance connected from the anode of the other of said systems to a source of anode potential and a common impedance connected between said cathodes and said point of zero potential, the input to said amplier being suiiicient to swing said grids beyond cut-on', the rea grid and a cathode, an input circuit coupled from the grid of one of said tubes to ground,v

an output impedance connected from the anode of the other of said tubes to a source of anode potential and a common impedance connected between said cathodes and ground, the remaining one of said grids being connected to ground and the remaining one of said anodes being connected to said source the input to said amplifier being suicient to swing said grids beyond cutolf.

5. A limiting amplifier comprising a pair of thermionic discharge tubes each having an anode, a grid and a cathode, an input circuit coupled from the grid of one of said tubes to ground, an anode impedance connected from the anode 0f the other of said tubes to a source of anode potential, a common impedance tween said cathodes and ground, one of said grids being connected to ground 'and the remaining one of said anodes being connected to said source, a wave shaping circuit coupled to said anode impedance and an output circuit coupled to said wave shaping circuit.

6. A limiting amplier comprising a pair of thermionic discharge tubes each having an anode, a grid and a cathode, an input circuit coupled to the grid of one of said tubes, impedance connected from the anode of the other of said tubes to a source of anode potential, a lcommon impedance connected between said cathodes and ground for vcoupling said cathodes together, the remaining one of said grids being connected to ground and the remaining one of said anodes being connected to the said source, the input to said ampliiier being suiiicient to swing said grids beyond cut-oli.

7. A limiting amplifier comprising a pair of thermionic discharge tubes each having an anode, a grid and a cathode, an input circuit coupled between the grid of one of said tubes and a point of zero reference potential an anode potential connection, a pair of resistances connected between said point and said anode potential connection, variable taps on each of said resistances, one of said taps being connected to each of said grids, an output impedance connected from the anode of one of said tubes to said anode potential connection, the remaining one of said anodes being connected directly to said anode potential connection and means for so coupling said cathodes together that a variation in the discharge current of one of said tubes thereby iniiuences the discharge current of the other of said tubes the input to said amplifier being suiiicient to swing said grids beyond cutoff.

8. A limiting amplifier comprising a pair of thermionic discharge systems each having an anode, a grid and a cathode, coupled between the grid of one of saidsystems and a point of zero reference potential, an anode potential connection, a pair nected between said point and said connection, variable taps on each of said resistances, one of said taps being connected to each of said grids, an output impedance connected from the anode of the other of said systems to said anode potential connection, the remaining one of said anodes being connected directly to said anode potential connection and means for vso coupling said an input circuit of resistances con-V cathodes together that a variation inthe discharge current of one of said systems thereby influences the discharge current of the other of said systems comprising a common connection from said cathodes to said point of Zero reference potential. the input to said amplifier being -suiilcient toswing said grids beyond cut-oil.

9. A limiting ampliiler comprising a pair of thermionic discharge systems each having an lanode, a grid and a cathode, an input circuit coupled between the grid or one of said systems and ground, an anode impedance connected from the anode of the other of said systems to a sourceA of anode potential, a connection from the anode of the iirst of said systems to said sourceof anode potential, an impedance connecting each oi' said cathodes to ground and means for so coupling said cathodes together that a variation in the discharge current of one of said systems thereby influences the discharge current of the other of said systems, the remaining one of said grids being connected to ground, the input to said ampliiler being suiiicient to swing said grids beyond cut-off, a wave shaping circuit coupled to said anode impedance and an output circuit coupled to said wave shaping circuit.

10. A limiting amplier comprising a pair of thermionic discharge systems each having an anode, a grid and a cathode means for so coupling the cathodes of said systems together that a variation in the discharge current of one of said a grid and a cathode, Ian input circuit coupled from the grid of one of said systems to ground,

the input to said amplifier being suiicient to' swing said grids beyond cut-oli, an output impedance connected from the anode of the other of said systems to a source of anode potential and a common impedance connected between said cathodes and gr'ound whereby said systemsare so coupled together that a variation in the dischargecurrent of one of said systems thereby influences the discharge currentot the other of said systems, the remaining grid and anode being maintained at zero signal potential.

12. An amplifier comprising a pair of thermionic discharge tubes each having an anode, a grid and a cathode, an input circuit coupled from the grid of one of said tubes to ground, the input to said amplifier being sufficient to swing said grids beyond cut-off, an output impedance connected from the anode oi the other of said tubes to a source of anode potential and a common impedance connected between said cathodes and ground, the Iremaining one of said grids being connected to ground and the remaining one of said anodes being connected to said source.

13, An amplifier comprising a pair of thermionic discharge systems each having an anode, a grid and a cathode, an input circuit coupled between the grid of one of said systems and' ground, the input to said amplifier being sufcient to swing said grid beyond cut-oil, a tuned output impedance connected from the ancde of the other of said systems to a source of anode potential, a common impedance connected between said cathodes and ground for so coupling said cathodes together that a variation in the discharge current of one of said systems thereby influences the discharge current of the other of said systems, the remaining one of said grids being connected to ground and the remaining one of said anodes being connected to the said source.

MURRAY G. CROSBY. 

